Superconductors conduct electricity with essentially no resistance below certain cryogenic temperatures, maximizing electrical efficiency. However, superconductors currently available for commercial application do not become superconducting until they have been cooled to temperatures near absolute zero, requiring an expensive and complicated helium refrigeration system. A helium refrigeration system can be used to provide cooling to about 4.2 K., which is the boiling point of helium. Also, due to the deformation characteristics of these superconductors, primarily niobium-titanium and niobium-tin alloys, the manufacture of superconducting wires or cables is difficult and expensive.
Certain high purity metals, which will be referred to hereafter as hyperconductors, exhibit exceptionally low electrical resistance at higher cryogenic temperatures which can be achieved with a liquid hydrogen refrigeration system. A liquid hydrogen cooling system can be used to provide cooling to about 20.3 K., the boiling point of liquid hydrogen. These materials are ideal candidates for space applications since liquid hydrogen is used as a propellant in spacecraft. The term hyperconductor refers to materials with very high (on the order of 5000 or greater) residual resistivity ratios (the ratio of bulk electrical resistivity at room temperature to that at 4.2 K.) and high Debye temperatures. A material with a high residual resistivity ratio generally has an exceptionally low residual resistivity at 4.2 K., with high purity metals having the lowest residual resistivities. The Debye temperature determines the rate at which resistivity increases with temperature from the residual resistivity at 4.2 K. The resistivity of metals with low Debye temperatures increases rapidly with temperature resulting in a relatively high resistivity at temperatures only slightly above the liquid helium boiling point of 4.2 K. However, preferred materials for use as hyperconductors have a high Debye temperature and their resistance increases gradually as the temperature increases from 4.2 K. such that their resistance at 20 K. remains extremely low.
The Debye temperature is an intrinsic property of a metal, i.e., it is relatively independent of impurities or structural defects in the metal. The residual resistivity is an extrinsic property, i.e., it is a function of purity, defect structure, microstructure, etc. An extremely low residual resistivity requires an ultra high purity metal on the order of 99.999% (5N), 99.9999% (6N), or higher. Thus, while many metals exhibit a very low residual resistivity at 4.2 K. when they are processed to extremely high levels of purity, most of these metals have a low Debye temperature such that the rapid rise in resistivity with only slight deviations in temperatures upwardly from 4.2 K. remove these metals from consideration as candidates for commercial application as hyperconductors due to their high resistance at temperatures achievable with liquid hydrogen refrigeration systems (on the order of 20 K.).
Hyperconductors can be divided into three general classes. Firstly, there are the soft hyperconductors like cadmium, tin, sodium and indium, which have low residual resistivities, are easy to purify, but have low Debye temperatures, A second class is hard, brittle hyperconductors such as beryllium, ruthenium, and chromium. While these metals have very high Debye temperatures, the chemistry and metallurgy of these metals is such that it is very difficult to process them to the high purities required to achieve low residual resistivities. The most readily applicable class of hyperconductors is the group of hard, ductile hyperconductors which includes aluminum, magnesium, copper, calcium, and scandium.
Space power systems have created a new application for a conductor with an exceptionally low electrical resistivity at cryogenic temperatures achievable with a liquid hydrogen refrigeration system since liquid hydrogen is readily available in spacecraft due to its use as a propellant. The low residual resistivity at the currently achievable purity levels of aluminum and copper and their relatively high Debye temperatures make both aluminum and copper likely candidates for hyperconductor applications in space applications. While aluminum and copper at purities of 99% can be used as hyperconductors, higher purities, such as 99.999% and 99.9999%, are preferred. Significant work has been done on the chemical and metallurgical processing of ultrapure aluminum and copper. However, aluminum hyperconductor is particularly interesting due to its low weight and exceptional magneto-resistance.
Hyperconductor aluminum is of an extremely high purity, resulting in its mechanical strength being very low and requiring a strengthening mechanism. An obvious solution is the use of a strengthening matrix surrounding the hyperconductor. This arrangement, whereby the high purity aluminum is embedded in a matrix, is also advantageous in reducing eddy current losses in the conductor if the resulting conductor is twisted, and the matrix resistivity is sufficiently large. Application of conventional high strength aluminum alloys as the matrix result in contamination of the high purity aluminum during high thermal excursions experienced during processing. This contamination may be avoided to a great extent through the use of dispersion strengthened aluminum alloys. However, a significant disadvantage of using such an aluminum alloy matrix with an aluminum hyperconductor for alternating current applications is the low resistivity of these alloys and the resulting high AC transverse magnetic field loss. The eddy currents induced by a changing transverse magnetic field tend to flow along the axis of the conductor; but, if a mulifilament conductor is twisted, the induced current is forced to flow through the matrix. In particular, twisting of the filaments will result in a transverse component of induced current through the matrix, where most of the loss occurs. What is needed is a hyperconductor for AC applications which is lightweight and overcomes the transverse magnetic field loss due to circulating currents or eddy currents.